1. Field of the Invention
The subject invention is concerned with solar collectors of the type in which a flat planar material containing a luminescent substance is used to absorb incident solar radiation which is then emitted by the luminescent material in the form of radiation of a lower frequency and in which the emitted radiation is transported to an exiting edge by internal reflection.
2. Prior Art
There are numerous publications and patents which deal with the conversion of solar energy to different wavelengths by means of luminescent or fluorescent materials. Among these publications and patents, specific reference is made to the following: W. H. Weber et al, Applied Optics, Volume 15, pages 2299-2300 (1976); Levellett et al, Applied Optics, Volume 16, pages 2684--2689; Reisfeld et al, Nature, Volume 274, pages 44-145 (1978); Goetzberger et al, Applied Physics, Volume 14, pages 123-139; Pittsburger Proceedings, 2nd European Community Photovoltaic Solar Energy Conference Berlin. Apr. 23-26, 1979; Reidel Publishing Company, pages 515-523 (1979); U.S. Pat. No. 4,110,123; U.S. Pat. No. 4,146,790; U.S. Pat. No. 4,149,902; and U.S. Pat. No. 4,190,465.
Basic to the solar energy concentrators of the above mentioned references is a light conducting medium in the form of a flat planar material having two opposing parallel surfaces of extended area relative to the edge surfaces of the medium. This planar layer typically consists of glass or plastic and contains a luminescent material, i.e., a luminescor, which absorbs solar radiation, thereby exciting the luminescor. The excited luminescor then emits electromagnetic radiation at a longer wavelength with an isotropic distribution of direction of propagation. Since a large fraction of the emitted radiation will reach the surface of the planar layer at an angle which is greater than the critical angle, the radiation will be trapped within the layer by internal reflection. This trapped radiation can only escape through an edge of the planar layer. In this manner, the solar radiation falling on the large major surface of the planar concentrator is emitted in a concentrated form through an edge.
The efficiency of such collectors is impacted by numerous factors. As is suggested, for example, in Weber et al, Applied Optics, Volume 15, page 2330 (1976), in order to prevent degradation of the total internal reflection of such a collector, the collector should be kept in an enclosed container to keep its surfaces free of any scattering or absorbing contaminants.
Other factors which affect the achievable efficiency for a luminescent solar collector include the radiation reabsorbed by the luminescor in the fluorescent layer, as well as transmission losses of the basic glass or plastic material containing the luminescor, and nonradiative losses such as photo thermal conversion and photochemical degradation.
As a practical matter, efficiencies only in the order of 1% to 15% have been achieved with luminescent solar collectors having concentration ratios ranging from about 4 to 20. Thus, there remains an apparent need to develop luminescent solar collectors that have higher efficiencies, for example, efficiencies in the order of 15% to 40% with concentration ratios of 20 to 10,000.